1. Parts of the Eye and their Functions
We are equipped with one of the most incredible optical instruments around: the eye. Before the way it works can be discussed in relation to physics, it is first necessary to get a little more acquainted with the parts of it that are involved. Following is a cross-sectional diagram of an eye, with numbers indicating the different parts, also outlined below:
1. The cornea, made out of a fairly dense, jelly-like material provides protection for the eye, and seals in the aqueous humor. It also provides most of the power of the eye (59 Dioptres), having about 46 Dioptres.
2. The aqueous humor, a watery liquid, helps keep the cornea in a rounded shape, similar to that of a lens.
3. The iris controls the amount of light that enters the eye. The amount of light is one of the factors determining how focused an image is on the retina.The brighter the light the eye is exposed to, the smaller the iris opening, or pupil, will be. The iris is the colored part of the eye as seen from the outside.
4. The lens controls the bending of light rays to a focused image on the retina. The shape of the lens changes by a process called accomodation, which is carried out by the ciliary muscles.
5. The ciliary muscles control the thickness of the lens. By contracting, or squeezing it, they make the lens thicker, and vice versa. Because the power of the lens is directly related to its thickness, ciliary muscles necessarily change the power of the lens by their movement. Therefore, it is also these muscles that control the focusing of the eye itself.
6. The vitreous humor is a jelly-like substance that helps the eye keeps its round shape. It is very close in optical density to the lens material.
7. The retina is the light-sensitive inner lining of the eye. It is black, which prevents any light rays that hit it from reflecting and thereby changing the image you see. The retina itself is thought capable of changing according to the amount of light it receives as well. The pupils of any persons eyes are always black, because they are simply a hole through which the retina can be seen.
8. The yellow spot is a small area on the retina where the sharpest images, that is, the finest detail, can be seen.
9. The optical nerve, attached to the retina, transmits the images received by the retina to the brain, effectively flipping them in the process (you will see why this is necessary in the following section). The part of the eye where the optical nerve joins the retina is known as the blind spot because no images can be observed at this point.